All flex pipes have a braid cover outside, usually with stainless steel. However, there are 3 different types of flex pipes, based on the construction of the interior.

Non-Lined Flex Pipes
Used mostly on older cars, as the name suggests, theres no lining inside, which can cause extra "whistling noise" (ie: worse than the whistling noise you already get from some SS headers)

Braid Lined Flex Pipes
As name suggests, this type of flex pipes have an internal braid to cover the inside, this allows a lower noise level and a higher heat resistance, especially on engines with turbos.

Interlock-lined Flex Pipes
Those flex pipes have an interlock liner made from a stainless steel expandable hose instead of braid like above. This type of Flex pipe reduces noise level even further, and can tolerate even more movement in the exhaust system.

I've used a rather short one (~20cm overall) with interlock insides to join 4-2-1 stock d16a9 ex mani with the rest of the exhaust, exactly in the same place where stock graphite gasket is.
~20k km, no complains =)